Thin-film temperature-sensitive resistor material and production process thereof

ABSTRACT

Described are a thin-film temperature-sensitive resistor material which comprises, at a temperature-sensitive resistor portion, a mixed crystal of a nitride and oxide of a transition metal such as vanadium preferably, that represented by the formula: MN x  O y  wherein 0&lt;x&lt;1, and 2≦y≦13/6, simultaneously exhibits a high temperature coefficient of resistance and a low specific resistance at about room temperature, and has excellent sensitivity at about room temperature; and a process for the production of a thin-film temperature-sensitive resistor material, which comprises forming its temperature-sensitive resistor portion by using a gas-atmosphere composed mainly of a nitrogen gas preferably, a mixed gas composed of nitrogen, argon and oxygen, and has a flow rate ratio of nitrogen to oxygen (nitrogen/oxygen) of 14/1 to 23/1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a thin-film temperature-sensitive resistormaterial used for thermometry or infrared detection and a process forproducing the material.

2. Description of the Related Art

Thermometers and infrared ray sensors using a thin-filmtemperature-sensitive resistor material are conventionally known. FIG. 7is a schematic view illustrating the structure of an infrared ray sensorwhich uses a thin-film temperature-sensitive resistor material. Thissensor has a thin-film temperature-sensitive resistor material 8 betweenelectrodes 11 on a supporting film 14. Over the material 8, a protectivefilm 10 and an infrared absorption layer 9 are stacked, while aninfrared reflective film 13 is disposed below the supporting film 14 viaspace 12. Infrared ray energy incident from the upper side of thedrawing is converted to heat at the infrared absorption layer 9. Theresulting increase of temperature causes a change in the resistance ofthe thin-film temperature-sensitive resistor material 8. Infrared rayscan be detected by applying an electric current or voltage to thematerial 8 from the electrodes 11 and reading the change.

Metal oxides are mainly used as conventional thin-filmtemperature-sensitive resistor materials. Among them, a vanadium metaloxide (VMOx: M represents a metal and x stands for the oxidation number)which is an oxide with a transition metal vanadium (V), and anothermetal has been used as a material having a relatively high temperaturecoefficient of resistance (for example, Japanese Patent Laid-Open Nos.253201/1985, 95601/1982 and 52882/1989).

As a conventional process for producing a thin-filmtemperature-sensitive resistor material, for example, a film can beformed on a substrate by carrying out sputtering with vanadium orvanadium and another metal as a target in an argon gas, followed byreaction with an argon-oxygen mixed gas. Alternatively, a film can beformed by the sol-gel processing in which a metal alcohol substance isapplied to a substrate, followed by decomposition and removal of theorganic residue by thermal treatment. FIG. 5 is a schematic viewillustrating a process for producing a thin-film temperature-sensitiveresistor material by the above-described conventional sputteringprocess. A desired thin-film temperature-sensitive resistor material canbe formed over a substrate 5 by disposing, as illustrated in FIG. 5, thesubstrate 5 and target 6 in a sputtering chamber 4 equipped with gasinlet-ports 1,2 and an exhaust port 7, introducing an argon gas from thegas inlet port 1 and an argon-oxygen mixed gas from the gas inlet port2, and sputtering the target 6.

The performance of the thin-film temperature-sensitive resistor materialis expressed by a temperature coefficient of resistance a (%/K) which isa resistance change rate per degree of a temperature change. FIG. 6 is agraph illustrating temperature characteristics of the resistance of theconventional thin-film temperature-sensitive resistor material composedof vanadium oxide, said material having been formed by sputteringvanadium in an argon-oxygen mixed gas. Temperature characteristics ofthe resistance of such a thin-film temperature-sensitive resistormaterial are also shown in FIGS. 1 and 2 on page 3 of Japanese PatentLaid-Open No. 253201/1985 or Jerominek, et al., Optical Engineering, 32,2094 (1993), FIG. 1.

When used for the thermometry or infrared detection at the temperature(20 to 30° C.) near room temperature, a thin-film temperature-sensitiveresistor material is required to exhibit a high temperature coefficientof resistance around the room temperature. The thin-filmtemperature-sensitive resistor material as described above has, as shownin FIG. 6, a temperature coefficient of resistance as low as -2%/K atabout room temperature and is therefore insufficient in sensitivity.

When a material has a high temperature coefficient of resistance atabout room temperature, more specifically, when the material has atemperature coefficient of resistance higher than -2%/K at about roomtemperature as shown in Jerominek et al, Optical Engineering, 32,2094(1993), FIG. 1, the specific resistance value of the material alsobecomes high, exceeding 0.1 Ωcm. Since it is generally impossible toform the thin-film temperature-sensitive material thicker than 1000 Å, ahigh specific resistance value means nothing but a high resistance. Thenoise level generated in the material also becomes high. As a result, ifthe temperature coefficient of resistance at about room temperature isincreased, the noise level is inevitably increased so that thesensitivity can not be improved.

As described above, it is difficult for the conventional thin-filmtemperature-sensitive resistor material composed of a metal oxide tosimultaneously satisfy both the requirements for an increase in thetemperature coefficient and lowering in the specific resistance value.

SUMMARY OF THE INVENTION

The present invention has been made with a view to overcoming theabove-described problems of the prior art. An object of the presentinvention is to provide a thin-film temperature-sensitive resistormaterial which exhibits a high temperature coefficient of resistance atabout room temperature (20 to 30° C.) and a low specific resistance, andhas excellent sensitivity at about room temperature; and a process forproducing the material.

In one aspect of the present invention, there is thus provided athin-film temperature-sensitive resistor material, which comprises, at atemperature-sensitive resistor portion thereof, a mixed crystal of anitride and oxide of a transition metal.

In another aspect of the present invention, there is also provided aprocess for producing the above-described thin-filmtemperature-sensitive resistor material, which comprises sputtering atransition metal target in a gas-atmosphere composed mainly of anitrogen gas to form a mixed crystal of a nitride and oxide of atransition metal.

The present invention makes use of the properties that a transitionmetal oxide has both metallic and non-metallic properties and theseproperties separate into respective ones at a specific temperature as aborder line; the electric properties of a metal nitride are easilycontrollable by a nitrogen content; a transition metal can be readily inthe form of a mixed crystal of its oxide and nitride.

The transition temperature of a transition metal oxide from a metal to anon-metal is generally determined by the valence number of oxygen boundwith the metal. In the present invention, this transition metal is inthe form of a mixed crystal with a nitride which makes it possible tochange the transition temperature more minutely according to anitrogen/oxygen ratio bound with the metal. As a result, a thin-filmtemperature-sensitive resistor material which can simultaneously satisfya high temperature coefficient of resistance and a low specificresistance at about room temperature is available.

Accordingly, the thin-film temperature-sensitive resistor material ofthe present invention simultaneously shows a high temperaturecoefficient of resistance and a low specific resistance value at aboutroom temperature so that it is excellent in the sensitivity at aboutroom temperature and is therefore very useful for the applications suchas infrared ray sensor and temperature detector each used at about roomtemperature.

Furthermore, according to the process of the present invention, suchexcellent thin-film temperature-sensitive resistor material can beeasily and favorably produced.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate preferredembodiments and examples of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a production process of athin-film temperature-sensitive resistor material of the presentinvention.

FIG. 2 is a graph illustrating temperature characteristics of theresistance of each of the thin-film temperature-sensitive resistormaterials obtained in Examples 1 to 4.

FIG. 3 is a graph illustrating the composition distribution of thethin-film temperature-sensitive resistor material of Example 1 in thefilm-thickness direction by Auger electron spectroscopy.

FIG. 4 is a graph illustrating an X-ray diffraction of the thin-filmtemperature-sensitive resistor material of Example 1.

FIG. 5 is a schematic view illustrating a production process of theconventional thin-film temperature-sensitive resistor material.

FIG. 6 is a graph illustrating temperature characteristics of theresistance of the conventional thin-film temperature-sensitive resistormaterial.

FIG. 7 is a schematic view illustrating the structure of an infrared raysensor for which a thin-film temperature-sensitive resistor material hasbeen used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thin-film temperature-sensitive resistor material of the presentinvention is formed, at its temperature-sensitive resistor portion, of amixed crystal of a nitride and oxide of a transition metal. As thetransition metal, vanadium is preferred. In addition, transition metalsusable as a material for a transition metal oxide which is transitionalbetween a metal and non-metal, for example, chromium, manganese, ironand cobalt can be employed. The mixed crystal of the nitride and oxideof a transition metal is preferred to have a composition represented bythe following formula: MN_(x) O_(y) (M represents a transition metal) inwhich 0<x<1, and 2≦y≦13/6. The film thickness of the thin-filmtemperature-sensitive resistor material may be determined as desiredaccording to the application or the like, but about 100 to 1000 Å ispreferred.

The mixed crystal MN_(x) O_(y) has an oxide-like composition, but sinceit is a mixture with a nitride, oxidation or reduction hardly proceeds.It is however preferred to protect the temperature-sensitive resistorportion with a silicon nitride film, thereby favorably preventing theproperties of the material from changing with the passage of time. Theprotective film can be formed using a SiH₄ gas, NH₃ gas or nitrogen gas,for example, by the plasma chemical vapor phase deposition method.

The thin-film temperature-sensitive resistor material according to thepresent invention can be produced easily and favorably by thehigh-frequency sputtering method in which a transition metal target issputtered in a gas-atmosphere composed mainly of a nitrogen gas to forma mixed crystal of the nitride and oxide of a transition metal.

FIG. 1 is a schematic view illustrating a process for producing thethin-film temperature-sensitive resistor material of the presentinvention by the high-frequency sputtering method. The apparatus shownin FIG. 1 has a similar structure to that shown in FIG. 5 except that agas inlet port 3 is disposed for the introduction of a nitrogen gas.

For example, a vanadium oxynitride film can be formed on a substrate 5by disposing the substrate 5 and a target 6 in a sputtering chamber 4 asshown in FIG. 1, evacuating the sputtering chamber 4, heating thesubstrate 5, introducing an argon gas from a gas inlet port 1, anargon-oxygen mixed gas from a gas inlet port 2 and a nitrogen gas from agas inlet port 3, respectively, and starting discharge to effectsputtering of the target 6. The deposition rate can be controlled by aninputted power, a distance between the substrate 5 and the target 6, orthe like.

The sputtering gas is preferably a mixed gas of nitrogen, argon andoxygen as described above. It is more preferred that the flow rate ratioof nitrogen to oxygen (nitrogen/oxygen) falls within a range of from14/1 to 23/1. The argon gas only contributes to the sputtering of thetarget 6. Since it hardly composes the film, it does not have aninfluence on the nitrogen/oxygen ratio, which is a compositional ratioof the film. Accordingly, the compositional ratio of the film isdetermined by the flow rate ratio of nitrogen to oxygen. The oxygencontent in the argon-oxygen gas mixture may be determined as neededdepending on the nitrogen concentration (nitrogen gas flow rate) in thewhole gas-atmosphere.

Incidentally, it is difficult to add oxygen to the film by onlyintroducing an oxygen gas into the film, because the sputter particleshave already been nitrided. It is preferred to sputter vanadium in aninactive argon gas mixed with oxygen as described above, which makes itpossible to add oxygen to vanadium nitride smoothly.

Other sputtering conditions may be determined as needed depending on theperformance required for the resulting film. For example, a sputteringpressure, which will have an influence on the temperature coefficient ofresistance or a specific resistance of the film, is preferablycontrolled to about 4 to 30 mTorr.

It is also preferred to carry out preliminary sputtering of the target 6for a proper time prior to the above-described sputtering.

The present invention will hereinafter be described more specifically byexamples.

Examples 1-4

As shown in FIG. 1, disposed in a sputtering chamber 4 were a vanadiummetal as a target 6 and a silicon substrate as a substrate 5. Afterevacuation of the sputtering chamber 4 to 10⁻⁶ Torr or less, thesubstrate 5 was heated to about 300° C. and as a sputtering gas, anargon gas, an argon-oxygen gas mixture (oxygen content: 20%) and anitrogen gas were introduced from a gas inlet port 1, a gas inlet port 2and a gas inlet port 3, respectively.

Those gases were introduced simultaneously into the sputtering chamber 4and discharge was started, whereby the surface of the target 6 wassubjected to preliminary sputtering for 30 minutes. Then, the formationof a vanadium oxynitride film on the substrate 5 was started. Thefilm-forming conditions in each example are shown in Table 1. Inparticular, the ratio of a gas flow rate (nitrogen/argon/oxygen) waschanged within a range of from 14/4/1 to 23/4/1 by changing the flowrate of nitrogen.

To the vanadium oxynitride film having a thickness of 1000 Å thusproduced, electrodes were attached, whereby temperature characteristicsof the resistance were measured. The resistance at the measuringtemperature in each of Examples is shown in Tables 2 to 5. FIG. 2, inwhich measuring results are summarized, is a graph illustratingtemperature characteristics of the resistance of a thin-filmtemperature-sensitive resistor material of each Example composed of avanadium oxynitride film. Table 6 shows a temperature coefficient of theresistance of the thin-film temperature-sensitive resistor materialobtained in each Example.

A vanadium oxide film used for the conventional thin-filmtemperature-sensitive resistor material has a specific resistance at 25°C. of about 0.1 Ωcm and a temperature coefficient of resistance of -2.3%/K. The thin-film temperature-sensitive resistor materials, which hadbeen obtained in Examples 1 to 4 and were composed of a vanadiumoxynitride film, had a specific resistance at 25° C. as small as 0.0026to 0.077 Ωcm, showed a markedly large temperature change of resistanceat about room temperature as shown in FIG. 2, and had a temperaturecoefficient as high as -3.9 to -7.2%/K as shown in Table 6. In short,the thin-film temperature-sensitive resistor materials obtained inExamples 1 to 4 had a good sensitivity at least twice as high as that ofthe conventional one at about room temperature.

Next, the composition of the thin-film temperature-sensitive resistormaterial obtained in Example 1, which material was composed of avanadium oxynitride film, in the film thickness direction was evaluatedby Auger electron spectroscopy. FIG. 3 is a graph illustrating thecompositional distribution of the material in the film thicknessdirection. The film had a composition of VN_(x) O_(y) wherein 0<x<1 andy=2. The thin-film temperature-sensitive resistor materials obtained inExamples 2 to 4 showed almost the same measuring results, because theirflow rate ratios were the same in the number of figures with that ofExample 1.

In addition, the crystallinity of the thin-film temperature-sensitiveresistor material of Example 1 composed of a vanadium oxynitride filmwas studied by X-ray diffraction. FIG. 4 is a graph illustrating itsX-ray diffraction. From the peaks, it has been found that nitrogen wascontained in VO₂ to V₆ O₁₃. Accordingly, the material has a compositionfalling within a range of 0<x (nitrogen)<1 and 2≦y (oxygen)≦13/6.

Then, a protective film 10 (FIG. 7) which was a silicon nitride film andhad a film thickness of 3000 Å was formed over the thin-filmtemperature-sensitive resistor material by using SiH₄ and NH₃ gases at asubstrate temperature of 250° C. and pressure of 100 pa in accordancewith the plasma chemical vapor deposition method. The formation of theprotective film 10 has made it possible to favorably prevent atime-dependent change in the properties of the thin-filmtemperature-sensitive resistor material.

While preferred embodiments and examples of the present invention havebeen described using specific terms, such description is forillustrative purpose only, and it is to be understood that changes andvariations may be made without departing from the spirit or scope of thefollowing claims.

                  TABLE 1                                                         ______________________________________                                                  Flow    Flow rate of                                                                            Oxygen/                                           Sputtering                                                                              rate of argon &   nitrogen                                                                             RF                                         pressure  nitrogen                                                                              oxygen    flow rate                                                                            input                                                                              Sputtering                            (mTorr)   (sccm)  (sccm)    ratio  (W)  time (min)                            ______________________________________                                        Ex. 1                                                                              16       22      4.8     0.044  500  80                                  Ex. 2                                                                              16       13      4.8     0.074  500  95                                  Ex. 3                                                                              16       18      4.8     0.053  500  95                                  Ex. 4                                                                              16       20      4.8     0.048  500  95                                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Ex. 1: Measuring                                                              temperature (° C.)                                                                    Resistance (Ω)                                           ______________________________________                                        22.5           425.5                                                          24.24          387.5                                                          28.48          289.9                                                          33.31          199.4                                                          38.25          141.8                                                          43.19          109.8                                                          43.17           92.9                                                          ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Ex. 2: Measuring                                                              temperature (° C.)                                                                    Resistance (Ω)                                           ______________________________________                                        20.91          2485.4                                                         24.27          2058.4                                                         28.66          1421.2                                                         33.49           798.4                                                         38.37           416.3                                                         43.29           231.5                                                         48.25           147.3                                                         ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Ex. 3: Measuring                                                              temperature (° C.)                                                                    Resistance (Ω)                                           ______________________________________                                        20.91          6085.3                                                         24.27          5277.5                                                         28.66          4025.9                                                         33.49          2498.3                                                         38.37          1306.9                                                         43.29           605.7                                                         48.25           277.6                                                         ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Ex. 4: Measuring                                                              temperature (° C.)                                                                    Resistance (Ω)                                           ______________________________________                                        22.26          48778                                                          24.18          46554                                                          28.32          41183                                                          33.16          35410                                                          38.1           29128                                                          43.04          21601                                                          48.0           12458                                                          ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                                     Temperature coefficient at                                                    25° C. (%/K)                                              ______________________________________                                        Example 1      -6.5                                                           Example 2      -7.2                                                           Example 3      -5.3                                                           Example 4      -3.9                                                           Conventional Example                                                                         -2.3                                                           ______________________________________                                    

What is claimed is:
 1. A thin-film temperature-sensitive resistormaterial, which comprises, at a temperature-sensitive resistor portionthereof, a mixed crystal of vanadium oxynitride, which has a temperaturecoefficient of -3.9 to -7.2%/K, wherein said mixed crystal has acomposition represented by the following formula: MN_(x) O_(y), whereinM represents vanadium and wherein 0<x<1, and 2≦y≦13/6.
 2. A materialaccording to claim 1, wherein said temperature-sensitive resistorportion is protected with a silicon nitride film.
 3. The materialaccording to claim 1, wherein said film has a film thickness of 100 to1000 Å.